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Check the Calibration of Your Sphyg

Written By: 
Michael Falco / Quality Manager

In previous posts, we have reviewed the 10 Steps to Accurate Manual Blood Pressure Measurement and 10 Factors That Can Affect Blood Pressure Readings. As important as these details are, accurate measurement also requires two things:

  • A trained clinician who takes the time to follow the 10 steps and think about the 10 factors that go into an accurate BP measurement
  • Capable, well-maintained equipment that is available and easily-accessible, be it an automated monitor or a sphygmomanometer for manual measurement.

Researchers at Oxford University recently studied the second element above and found that nearly 14% of the instruments used to measure BP were not calibrated correctly. This study has been repeated by different groups around the world with largely similar results. The take-home message: equipment is as important as the clinician to taking a BP measurement accurately. So make sure you buy an instrument that meets the accuracy requirements mandated by law, and have the calibration checked periodically to ensure it remains accurate.

Aneroid devices provide a simple way to verify if they are OUT of calibration. For those marketed in the US, there is an oval or a small rectangle at the bottom of the gauge – often referred to as the zero point. If the indicator needle falls OUTSIDE the oval when the unit is fully deflated (preferably disconnected from the inflation system), the unit is OUT of calibration.

With needle outside the oval, you can be 100% sure the unit is OUT of calibration
With needle inside the oval, you can NOT be 100% sure the unit is in calibration

However the reverse isn’t necessarily true. That is, if the needle is within the oval the gauge may still be out of calibration. That is why it is important to purchase a recognized brand AND have your aneroid instrument periodically checked against a reference standard – a manometer of known accuracy. The prevailing standards recommend instruments be checked a minimum of once every two years.

Mercury sphygs are accurate so long as the meniscus of the mercury column is within 3mmHg of the zero point when the unit is fully deflated.

Most automated BP (consumer or clinical grade) instruments do a calibration self check during power up and will notify the operator if calibration is needed.

Bottom line – spend a little more for a reputable brand and have it checked according to the manufacturer’s recommendations.